The present invention relates to a soybean-based adhesive resin, composites products made with that adhesive, and a method of manufacturing such adhesives and composites products.
Urea-formaldehyde (UF) resins and other formaldehyde-condensed resins, which are toxic petroleum-based adhesives, have been used as wood adhesives for many years. However, the level of formaldehyde gas emission from UF-bonded products is heavily regulated by law, particularly with respect to indoor use. In addition, the high cost of most formaldehyde-condensed resins and the poor moisture resistance of UF resins have resulted in the need for alternative wood adhesives.
There is a growing interest to develop wood adhesives from renewable substances to reduce the dependency on petroleum-based chemicals and to expand the non-food use of agricultural commodities. In particular, uncertainty in future supplies of petroleum-derived chemicals and stringent regulations on toxic emissions from building materials bonded with certain synthetic resins have compelled the forest products industry to reevaluate wood adhesives from renewable substances. The agriculture industry also is eager to invest in researching nonfood industrial uses of agricultural products to expand their markets. A significant challenge is to develop resin adhesives from renewable substances meeting stringent performance requirements at reasonable costs.
As an alternative to petroleum-based adhesives, protein glues, such as casein, blood, and soy glues, reached their peak use in the 1960s and faded out in the 1970s. Most of the important research on formulating wood adhesives with protein occurred before 1960.
U.S. Pat. No. 2,817,639 to Ash et al. disclosed a plywood glue containing high blood solids blended with a commercial phenol-formaldehyde (PF) resin and sodium silicate. This required a specific mixing procedure to avoid a high viscosity problem. Lambuth, xe2x80x9cBlood Glues,xe2x80x9d in Handbook of Adhesives, I. Skeist, ed., Reinhols Publishing Corp., NY (1962) discloses the interactions between different types of PF resins and blood albumin. PF resins of simple structure behave much the same as aromatic alcohols, causing a change in blood glue consistency from gelatinous to granular, often accompanied by a reduction in viscosity. Alkaline phenolic resins of moderate complexity do not disperse blood proteins to a great extent. Highly complex and reactive PF resins, however, cause gelation of blood proteins, creating a major viscosity problem at any mixing combination. U.S. Pat. No. 2,368,466 to Golick et al. disclosed an exterior phenolic plywood glue containing up to 70% dried blood. This glue was prepared by first reacting cresol and casein with formaldehyde at room temperature to form an emulsion. This reaction was catalyzed by NH4OH. Subsequently, this emulsion was mixed with blood albumin at room temperature to form a smooth glue. The casein did not react with PF resins, and the fluid glue contained finely precipitated particles of blood albumin-phenol-aldehyde complex.
U.S. Pat. No. 3,153,597 to Weakley et al. disclosed moisture resistant plywood adhesives by cross-linking casein with dialdehyde starch.
U.S. Pat. No. 5,593,625 to Riebel et al. describes methods of preparing a legume-based thermosetting resin for producing rigid biocomposite materials. This resin was formulated by cross-linking soy flour with methyl diphenyl isocyanate and mixing the resin with paper fibers in a ratio about 4:6. A molded composite product was produced.
U.S. Pat. No. 5,371,194 to Ferretti describes a method of synthesizing a thermosetting resin from a mixture of protein and carbohydrate by ammoniation. This ammoniated resin was developed as a moisture resistant binder for pelletizing coal fines.
Further, the use of soy protein isolates and phenol-resorcinol-formaldehyde (PRF) in combination for lumber finger jointing is currently under mill trial studies. In this finger jointing system, the soy protein isolates and PRF are separately applied onto different fingers, and upon joining these fingers, interactions between soy protein and PRF first immediately cause the adhesive to gel and, eventually, the adhesive is completely cured in several hours.
Although great effort has been undertaken to develop protein glues, such glues were slow curing and had poor moisture resistance. As a result, the art sought to develop synthetic resin adhesives. Detlefsen, xe2x80x9cBlood and Casein Adhesives For Bonding Wood,xe2x80x9d in Adhesives from Renewable Resources, Hemingway et al., eds., ACS Symposium Series 385, Am. Chem. Soc., Washington, D.C. (1989).
The present invention is directed to overcoming the deficiencies in protein glues and their use in manufacturing composite products.
One aspect of the present invention relates to a soybean-based adhesive resin including a soybean flour and a cross-linking agent. The cross-linking agent is reacted with the functional groups in the soybean flour to form an adhesive resin.
The present invention also relates to a method of making a soybean-based adhesive resin. This method involves providing an aqueous solution of soybean flour and adding a cross-linking agent to the solution. The cross-linking agent is added to the solution under conditions effective to cross-link the soybean flour and the cross-linking agent to form an adhesive resin.
Another aspect of the present invention relates to a composite product which includes particulate plant material and the soybean-based adhesive resin. The adhesive is prepared by reacting the cross-linking agent with functional groups in the soybean flour.
The present invention also relates to a method of making a composite product. This method involves providing particulate plant material and applying the soybean-based adhesive resin to the particulate plant material. The adhesive is prepared by reacting the cross-linking agent with the functional groups in the soybean flour.
The soybean-based adhesive resin of the present invention provides a low-cost wood adhesive from a renewable resource as an alternative to petroleum-based adhesives. This resin can be used as an exterior adhesive in the manufacture of wood composite panel products and can replace urea-formaldehyde resins for interior fiberboard, alleviating formaldehyde emissions and poor moisture resistance problems.
The present invention relates to a soybean-based adhesive resin including a soybean flour and a cross-linking agent. The cross-linking agent is reacted with the functional groups in the soybean flour to form an adhesive resin.
Preferably, the soybean flour contains more proteins than carbohydrates.
Most preferably, the soybean flour is defatted. Defatted soybean flour is commercially available and is obtained by grinding soy flakes after hexane extraction for soy oil. After hexane extraction, defatted soybean flour typically has the following moisture-free chemical composition:
Most preferably, after soy oil extraction, defatted soybean flour has the following moisture-free average chemical composition:
The cross-linking agent reacts with the functional groups in the soybean flour to form an adhesive resin. Suitable cross-linking agents include phenol-formaldehyde resin, melamine-urea-formalde resins, and mixtures thereof.
The preferred cross-linking agent is a phenol-formaldehyde (PF) resin. In this embodiment, the hydroxymethylol groups (xe2x80x94CH2OH) of the PF resin react with the functional groups of the soybean protein, such as tyrosine and hydroxyl groups (xe2x80x94OH), to form methylene or ether linkages between the soy protein and the PF resin. In addition, the hydroxymethylol groups of the PF resin react with the hydroxyl groups of the soybean carbohydrates to form ether linkages. These condensation reactions between the PF resin and protein and carbohydrates in soy flour occur particularly during curing stages of the soybean-based adhesive at high temperatures. As a result of the condensation reactions, a cross-linked product is formed. More specifically, the hydroxymethylol groups of the PF resin condense with the soy protein forming methylene (xe2x80x94CH2) and ether (xe2x80x94CH2O) linkages, and condense with the soybean carbohydrates forming ether linkages to form the cross-linked product.
Soybean flour, since it contains a relatively large amount of carbohydrates, is not compatible with alkaline PF resins. Specifically, during curing at high temperatures, alkaline PF resins transform carbohydrates into saccharic acids which interfere with polymerization of the PF resin and inhibit condensation reactions between the PF resin and soy flour. However, under the neutral condition the carbohydrates in soybean flour are not converted to saccharic acids. Therefore, the hydroxymethylol groups in the PF cross-linking agent are able to condense with themselves and with the many functional groups in the soybean protein and hydroxyl groups in the soybean carbohydrates to form a cross-linked product.
Soybean flour is a good natural adhesive, but the adhesive bonds so formed are not moisture resistant. Therefore, any level of cross-linking of soybean flour with a PF resin improves the moisture resistance of the adhesive bonds due to the reduced water solubility of cross-linked soybean flour. Preferably, the weight ratio of soybean flour to cross-linking agent in the soybean-based adhesive resin of the present invention is from about 8:2 to about 6:4. Most preferably, the weight ratio of soybean flour to cross-linking agent is about 7:3. The physical properties of wood composite products bonded with such soybean-based adhesive resins are comparable to those of products bonded with pure PF resins; however, the cost of pure PF resins is about three times greater than soybean flour. Therefore, the soybean-based adhesive resin of the present invention is more economical than pure PF resin.
The present invention also relates to a method of making a soybean-based adhesive resin. This method includes providing an aqueous solution of soybean flour and adding a cross-linking agent under conditions effective to cross-link the soybean flour and the cross-linking agent to form an adhesive resin.
The soybean flour may be prepared by methods known to those skilled in the art. A preferred method for preparing the soybean flour solution of the present invention includes dissolving sodium bisulfite in water, adjusting the pH to from about 6.8 to 7.1 with sodium hydroxide, heating the solution to from about 45xc2x0 C. to about 55xc2x0 C., adding defoamer, and adding dry soy flour under conditions effective to produce a homogenous solution. Preferably, the solution is heated to about 50xc2x0 C. Addition of sodium bisulfite during preparation of soy flour solution partially depolymerizes the soy protein by cleaving the disulfide linkages. Cleavage of disulfide bonds reduces the viscosity of the soy solution. Maintaining the soy flour solution at the neutral pH and 50xc2x0 C. obtains a high solubility of soy flour and avoids protein gelation. Adding defoamers reduces foaming of soy flour solution, making preparation of the soy flour solution easier to handle. Preferred defoamers include cedarwood oil, Sigma antifoam 204, or pine oil. Most preferably, the defoamer is cedarwood oil. Preparation of soy flour in this manner produces a solution at a solid content as high as possible and at a viscosity manageable in the subsequent resin formulation.
A preferred method for preparing the cross-linking agent of the present invention includes combining phenol, formaldehyde, and sodium hydroxide to form a mixture, heating the mixture, refluxing the mixture, and neutralizing the mixture to a pH of about 6.9 to about 7.1. Most preferably, the pH is about 7.
The soybean-based adhesive resin is prepared by cross-linking the proteins and carbohydrates in the soybean flour with the cross-linking agent. Preferably, the cross-linking reaction is carried out at 50xc2x0 C. by vigorously blending the soybean flour solution and the cross-linking agent.
The soybean-based adhesive resin of the present invention may be prepared in liquid or powder form. In one preferred embodiment, the powder form of the soybean-based adhesive resin of the present invention is prepared by freeze-drying the soybean-based adhesive resin produced by the claimed method and grinding the freeze-dried soybean-based adhesive resin under conditions effective to form a powder soybean-based adhesive resin. In another preferred embodiment, the powder form of the soybean-based adhesive resin of the present invention can be manufactured with a spray drier, resulting in a powder soybean-based adhesive resin of better quality, especially with respect to uniformity in particle size. The powder soybean-based adhesive resin of the present invention is preferred, because it has a long storage life if properly stored.
Another aspect of the present invention relates to a composite product which includes particulate plant material and the soybean-based adhesive resin of the present invention.
Suitable particulate plant materials include wood flakes, wood fibers, plant fibers, and a combination of wood and plant fibers. Useful plant fibers include wheat straw fibers, rice fibers, switchgrass fibers, soybean stalk fibers, bagasse fibers, cornstalk fibers, and mixtures thereof. Preferably, wood fibers are used in combination with plant fibers. The preferred weight ratio of wood to plant fibers when used in combination is from about 2.5:7.5 to about 7.5:2.5, preferably about 5:5. Wood flakes may be produced by procedures known to those skilled in the art (Koch, xe2x80x9cUtilization of Hardwoods Growing on Southern Pine Sites,xe2x80x9d Vol. II, USDA Forest Service, Agriculture Handbook No. 605 (1985), which is hereby incorporated by reference). Fiber furnishes containing wood fiber, plant fiber, or a combination thereof may be produced by procedures known to those skilled in the art (Suchsland et al., xe2x80x9cFiberboard Manufacturing Practices in the United States,xe2x80x9d USDA Forest Service, Agriculture Handbook No. 640 (1986), which is hereby incorporated by reference). Preferably, wood and plant fiber furnishes are produced by a pressurized double-refining process as described in Example 4. Cornstalk fibers and other plant fibers may also be produced with an atmospheric disk refiner as outlined in Kuo et al., xe2x80x9cProperties of Wood/Agricultural Fiberboard Bonded with Soybean-based Adhesives,xe2x80x9d For. Prod. J., 48:71-75 (1998), which is hereby incorporated by reference.
In another embodiment, the composite product of the present invention further includes a wax emulsion. Wax emulsion or slack wax is used in producing composite panels to improve moisture resistance or, more specifically, to reduce water absorption and water vapor adsorption. For example, a suitable wax emulsion is Cascowax EW-403H (Borden Chemical, Inc., Springfield, Oreg.).
The soybean-based adhesive resin of the present invention is particularly useful for preparing wood composite panel products such as fiberboard, particleboard, plywood, and flakeboard. By utilizing these adhesive resins, moisture resistant and toxic emission-free wood composite products can be produced. In particular, the soybean-based adhesive resin can replace urea-formaldehyde (UF) resin for the production of fiberboard panels for interior applications to alleviate the formaldehyde emission and poor moisture resistance problems. Alternatively, the resin can be used as exterior adhesive resin for the production of construction grade wood composites at a cost of about 50 percent less than phenol-formaldehyde (PF) resin.
Another aspect of the present invention relates to a method of making a composite product. This method involves providing particulate plant material and applying the soybean-based adhesive resin to the particulate plant material. The adhesive is prepared by reacting the cross-linking agent with the functional groups in the soybean flour.
The soybean-based adhesive resin of the present invention may be applied to the particulate plant material by spraying or mechanical mixing in liquid form, or it may be applied in powder form. Preferably, the soybean-based adhesive resin is applied in liquid form to the particulate plant material by a nozzle atomizer or by a spinning-disk atomizer. In another embodiment, the soybean-based adhesive resin is applied to the particulate plant material by first spraying the particulate plant material with the liquid soybean-based adhesive resin of the present invention and then applying a powder form of the soybean-based adhesive resin of the present invention to the particulate plant material. The liquid soybean-based adhesive is used in combination with the powder soybean-based adhesive to reduce moisture content of the particulate plant material. Preferably, the particulate plant material is sprayed with 30% of the desired amount of the soybean-based adhesive in liquid form and the remainder 70% of the desired amount of the soybean-based adhesive in powder form.
The soybean-based adhesive coated particulate plant material is spread to form a mat by procedures known to those skilled in the art (Maloney, Modern Particleboard and Dry-Process Fiberboard Manufacturing, Miller Freeman Publications, San Francisco, Calif. (1997), which is hereby incorporated by reference). The formed mat is consolidated to a predetermined thickness with a sufficient pressure at a temperature ranging from about 170xc2x0 C. to about 190xc2x0 C. for about 7 to about 15 minutes to obtain the products. Preferably, the mat is pressed at about 190xc2x0 C. Preferable press time depends on moisture content of the mat. For the mat formed from the particulate plant material sprayed with the soybean-based adhesive in liquid form, a high moisture content in the mat requires a long press time. When the soybean-based adhesive is applied to the particulate plant material in the combination of liquid and powder form, a short press time is sufficient to consolidate the mat. When used in powder form, the soybean-based adhesive resin is mixed with the particulate plant material, and the mixture is filled in a mold and consolidated under a high temperature and pressure to produce shaped composite products. In particular, the powder soybean-based adhesive resin is mixed with the particulate plant material in a weight ratio ranging from about 3:7 to about 2:8, preferably from about 2.5:7.5. The mixture is filled in a mold and consolidated at from about 175xc2x0 C. to about 200xc2x0 C., preferably 190xc2x0 C., for from about eight to about twelve minutes, preferably ten minutes, to obtain compression molded products.